Analytical solution to longitudinal settlement of segments of subsea shield tunnels in fault fracture zones and its application

The settlement control of tunnels in fault fracture zones is one of the key problems in the construction of subsea shield tunnels. The differential settlement of the tunnels induces the opening and dislocation of circumferential segment joint, increase of bolt stress and even leakage, which seriously affect the stability of tunnel structure and operation safety. Aiming at the longitudinal settlement of the subsea shield tunnels in the fault fracture zone stratum, through the analysis of the whole process of the tunnels crossing the fault fracture zone stratum, the theoretical formula for the normalized overlying loads is proposed, and the Taylor series expansion method is used to simplify the complex overburden loads into a multi-segment cubic polynomial function within 5% error. Combined with the Timoshenko beam theory, the analytical solution of the longitudinal settlement and internal force of the subsea shield tunnel segments is derived. Finally, the theoretical model is verified by an engineering example. The research shows that the settlement curve of the tunnel in the fault fracture zone shows surge section, slow increase section and sudden decrease section. The greater the dip angle of the fault fracture zone, the greater the differential settlement near the interface between the fault fracture zone and the normal surrounding rock. The fault fracture zone has a great influence on the settlement of the tunnels within the width of one time the fracture zone at the extension side. With the increase of the width of the fault fracture zone, the peak settlement of the tunnel in the fault fracture zone gradually increases and tends to be stable. When the width of the fault fracture zone exceeds 6 times the tunnel diameter, the peak settlement of the tunnel basically remains unchanged. The research results may provide a basis for the longitudinal settlement calculation and performance evaluation of subsea shield tunnels in fault fracture zones.